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Feedback Process (feedback + process)
Selected AbstractsSensitivity of an Arctic regional climate model to the horizontal resolution during winter: implications for aerosol simulationINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2005Eric Girard Abstract Our ability to properly simulate current climate and its future change depends upon the exactitude of the physical processes that are parameterized on the one hand, and on model configuration on the other hand. In this paper, we focus on the latter and investigate the effect of the horizontal grid resolution on the simulation of a month of January over the Arctic. A limited-area numerical climate model is used to simulate the month of January 1990 over a grid that includes the Arctic and sub-Arctic regions. Two grid resolutions are used: 50 km and 100 km. Results show that finer details appear for regional circulation, temperature, and humidity when increasing horizontal resolution. This is particularly true for continental and sea ice boundaries, which are much better resolved by high-resolution model simulations. The Canadian Archipelago and rivers in northern Russia appear to benefit the most from higher horizontal resolution. High-resolution simulations capture some frozen rivers and narrow straits between islands. Therefore, much colder surface air temperature is simulated over these areas. Precipitation is generally increased in those areas and over topography due to a better representation of surface heterogeneities when increasing resolution. Large-scale atmospheric circulation is substantially changed when horizontal resolution is increased. Feedback processes occur between surface air temperature change over heterogeneous surfaces and atmospheric circulation. High-resolution simulations develop a stronger polar vortex. The mean sea-level pressure increases over the western Arctic and Iceland and decreases over the eastern Arctic. This circulation leads to a substantial cooling of the eastern Arctic and enhanced synoptic activity over the Arctic associated with an intensification of the baroclinic zone. Aerosol mass loading, which is simulated explicitly in this model, is significantly altered by the grid resolution change with the largest differences in aerosol concentration over areas where precipitation and atmospheric circulation are the most affected. The implications of this sensitivity study to the evaluation of indirect radiative effects of anthropogenic aerosols are discussed. Copyright © 2005 Royal Meteorological Society. [source] Photoperiodic differences in a forebrain nucleus involved in vocal plasticity: Enkephalin immunoreactivity reveals volumetric variation in song nucleus lMAN but not NIf in male European starlings (Sturnus vulgaris)DEVELOPMENTAL NEUROBIOLOGY, Issue 11 2010Tyler J. Stevenson Abstract Seasonal variation in the volume of various song control nuclei in many passerine species remains one of the best examples of naturally occurring adult neuroplasticity among vertebrates. The lateral portion of the magnocellular nucleus of the anterior nidopallium (lMAN) is a song nucleus that is important for song learning and seems to be critical for inducing variability in the song structure that is later pruned via a feedback process to produce adult crystallized song. To date, lMAN has not been shown to exhibit seasonal changes in volume, probably because it is difficult to resolve the boundaries of lMAN when employing histological methods based on Nissl staining. Here, lMANcore volumes were examined in intact photostimulated (i.e., breeding), castrated photostimulated and photorefractory (i.e., nonbreeding) male starlings (Sturnus vulgaris) to investigate the degree of seasonal variation in brain morphology. We present data demonstrating that the volumes of the total MAN and lMANcore delineated by enkephalin immunoreactivity are greater in photostimulated male starlings as compared to photorefractory males. Moreover, two other regions associated with the song system that have not been investigated previously in the context of seasonal plasticity namely (i) the medial portion of MAN (mMAN), and (ii) the nucleus interfacialis (NIf) did not display significant volumetric variation. We propose that greater lMANcore volumes are associated with the increase in vocal plasticity that is generally observed prior to production of stereotyped song. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 751,763, 2010 [source] Fire and the Miocene expansion of C4 grasslandsECOLOGY LETTERS, Issue 7 2005Jon E. Keeley Abstract C4 photosynthesis had a mid-Tertiary origin that was tied to declining atmospheric CO2, but C4 -dominated grasslands did not appear until late Tertiary. According to the ,CO2 -threshold' model, these C4 grasslands owe their origin to a further late Miocene decline in CO2 that gave C4 grasses a photosynthetic advantage. This model is most appropriate for explaining replacement of C3 grasslands by C4 grasslands, however, fossil evidence shows C4 grasslands replaced woodlands. An additional weakness in the threshold model is that recent estimates do not support a late Miocene drop in pCO2. We hypothesize that late Miocene climate changes created a fire climate capable of replacing woodlands with C4 grasslands. Critical elements were seasonality that sustained high biomass production part of year, followed by a dry season that greatly reduced fuel moisture, coupled with a monsoon climate that generated abundant lightning-igniting fires. As woodlands became more open from burning, the high light conditions favoured C4 grasses over C3 grasses, and in a feedback process, the elevated productivity of C4 grasses increased highly combustible fuel loads that further increased fire activity. This hypothesis is supported by paleosol data that indicate the late Miocene expansion of C4 grasslands was the result of grassland expansion into more mesic environments and by charcoal sediment profiles that parallel the late Miocene expansion of C4 grasslands. Many contemporary C4 grasslands are fire dependent and are invaded by woodlands upon cessation of burning. Thus, we maintain that the factors driving the late Miocene expansion of C4 were the same as those responsible for maintenance of C4 grasslands today. [source] Opportunistic multiuser scheduling with reduced feedback loadEUROPEAN TRANSACTIONS ON TELECOMMUNICATIONS, Issue 4 2010Yahya S. Al-HarthiArticle first published online: 27 MAY 2010 In this paper, we propose a reduced feedback opportunistic scheduling (RFOS) algorithm that reduces the feedback load while preserving the performance of opportunistic scheduling (OS). The RFOS algorithm is a modified version of our previously proposed algorithm, the DSMUDiv algorithm. The main difference is that RFOS consists of a probing process (search process) and a requesting feedback process based on a threshold. The threshold value is variable, and it depends on the probing process. To reduce the feedback rate, a quantised value indicating the modulation level is fed back, instead of the full value of the signal-to-noise ratio (SNR), which we call quantised SNR. The paper includes the closed-form expressions of the probing load, feedback load and spectral efficiency. In addition, we investigate the effect of the scheduling delay on the system throughput (STH). Under slow Rayleigh fading assumption, we compare RFOS algorithm with the DSMUDiv and optimal (full feedback load) selective diversity scheduling algorithms. Copyright © 2010 John Wiley & Sons, Ltd. [source] Understanding and optimizing multisource feedbackHUMAN RESOURCE MANAGEMENT, Issue 2 2002Leanne E. Atwater This article integrates the most recent research results on the topic of multisource feedback with what researchers have learned over the years about performance feedback in general. We believe that this review and set of recommendations represent the state of the art at this time. We provide practitioners with new ideas about how to continue to improve the multisource feedback process in their organizations. We also suggest ideas for feedback providers and facilitators about how to maximize the success of the feedback process. Additionally, we provide "food for thought" for researchers concerning ideas for future research. © 2002 Wiley Periodicals, Inc. [source] Numerical fluctuations in the northern short-tailed shrew: evidence of non-linear feedback signatures on population dynamics and demographyJOURNAL OF ANIMAL ECOLOGY, Issue 2 2002Mauricio Lima Summary 1,We studied a fluctuating population of the northern short-tailed shrew (Blarina brevicauda) in the Appalachian Plateau Province of Pennsylvania, USA, spanning 21 years of monitoring. We analysed the pattern of annual temporal variation fitting both time-series models and capture,mark,recapture (CMR) statistical models for survival and recruitment rates. 2,We determined that non-linear first-order models explain almost 80% of the variation in annual per capita population growth rates. In particular, a non-linear self-excited threshold autoregressive (SETAR) model describes the time-series data well. Average snowfall showed positive and non-linear effects on population dynamics. 3,The CMR statistical models showed that a non-linear threshold model with strong effects of population density was the best one to describe temporal variation in survival rates. On the other hand, population density or climatic variables did not explain temporal variation in recruitment rates. Survival rates were high during the study period. Weekly changes in population size attributable to new recruits entering in the population fluctuate between 21% and 0%, while the changes in population size related to survival fluctuate between 79% and 100%. 4,Two important results arise from this study. First, non-linear models with first-order feedback appear to capture the essential features of northern short-tailed shrew dynamics and demography. Secondly, climate effects represented by snowfall appear to be small and non-linear on this insectivore. The population dynamics of this shrew in the Appalachian Plateau are determined apparently by a strong non-linear first-order feedback process, which is related to survival rates. 5,This study links population dynamics and demography by detecting the underlying demographic mechanisms driving population dynamics. The feedback structure of this shrew suggests the existence of population dynamics dominated by intraspecific competitive interactions, such as aggression, solitary nesting, non-overlapping home ranges and territoriality. [source] Evaluative Feedback: Perspectives on Media EffectsJOURNAL OF COMPUTER-MEDIATED COMMUNICATION, Issue 2 2007Stephanie A. Watts Computer-mediated communication (CMC) makes it possible to deliver evaluative feedback, an essential component of learning, over distance. This article presents a theoretical model of the CMC-based evaluative feedback process from the perspective of both senders and receivers of feedback. Hypotheses suggested by the model were tested in a quasi-laboratory experiment with part-time MBA students comparing email with voicemail. Within matched-pair dyads, email senders, but not voicemail senders, viewed their negative feedback as being significantly more negative than did their receivers. Voicemail senders, but not email senders, reported significantly lower comfort levels than did their receivers. No feedback effectiveness differences were found between media conditions, although determinants of feedback effectiveness differed significantly depending on the medium. These results are generally consistent with the theoretical model. [source] Ants accelerate succession from mountain grassland towards spruce forestJOURNAL OF VEGETATION SCIENCE, Issue 4 2009Blanka Vlasáková Abstract Question: What is the role of mound-building ants (Lasius flavus) in successional changes of a grassland ecosystem towards a spruce forest? Location: Slovenské Rudohorie Mountains, Slovakia; ca. 950 m a.s.l. near the Obrubovanec point (1020 m a.s.l.; 48°41,N, 19°39,E). Methods: Both chronosequence data along a successional gradient and temporal data from long-term permanent plots were collected on ants, spruce establishment, and vegetation structure, together with additional data on spruce growth. Results: There are more spruce seedlings on ant mounds (4.72 m,2) than in the surrounding vegetation (0.81 m,2). Spruce seedlings grow faster on these mounds compared to surrounding areas. The first colonization wave of seedlings was rapid and probably occurred when grazing prevailed over mowing. Ant colony presence, mound volume, and plant species composition change along the successional gradient. Mounds become bigger when partly shaded but shrink in closed forest, when ant colonies disappear. Shade-tolerant acidophylic species replace grassland plants both on the mounds and in surrounding areas. Conclusions: The massive occurrence of Lasius flavus anthills contributes to a runaway feedback process that accelerates succession towards forest. The effect of ants as ecosystem engineers is scale-dependent: although they stabilize the system at the scale of an individual mound, they may destabilize the whole grassland system over a longer time scale if combined with changes in mowing regime. [source] Directional positive feedback and pattern at an alpine tree lineJOURNAL OF VEGETATION SCIENCE, Issue 1 2004Kathryn J. Alftine Lesica (2002) Abstract. The spatial pattern at alpine tree line may be part of a feedback process in which wind plays a central role. The basic aspects of such a feedback were embedded in a cellular automaton. Spatial metrics of the patterns generated by this simulation and those of observed patterns at a windy tree line site were ordinated using Principal Component Analysis. Only the simulations that included a directionally weighted feedback fell close to the observed sites in ordination space. MANOVA indicated that the directionally weighted feedback is most important in structuring the tree line pattern, but that random hotspots for establishment and the overall steepness of the environmental gradient from forest to tundra in space also have an effect. The importance of wind in determining feedback with the spatial pattern of a canopy indicates that nonlinear reactions to climatic change are likely. [source] Local recovery of Ca2+ release in rat ventricular myocytesTHE JOURNAL OF PHYSIOLOGY, Issue 2 2005Eric A. Sobie Excitation,contraction coupling in the heart depends on the positive feedback process of Ca2+ -induced Ca2+ release (CICR). While CICR provides for robust triggering of Ca2+ sparks, the mechanisms underlying their termination remain unknown. At present, it is unclear how a cluster of Ca2+ release channels (ryanodine receptors or RyRs) can be made to turn off when their activity is sustained by the Ca2+ release itself. We use a novel experimental approach to investigate indirectly this issue by exploring restitution of Ca2+ sparks. We exploit the fact that ryanodine can bind, nearly irreversibly, to an RyR subunit (monomer) and increase the open probability of the homotetrameric channel. By applying low concentrations of ryanodine to rat ventricular myocytes, we observe repeated activations of individual Ca2+ spark sites. Examination of these repetitive Ca2+ sparks reveals that spark amplitude recovers with a time constant of 91 ms whereas the sigmoidal recovery of triggering probability lags behind amplitude recovery by ,80 ms. We conclude that restitution of Ca2+ sparks depends on local refilling of SR stores after depletion and may also depend on another time-dependent process such as recovery from inactivation or a slow conformational change after rebinding of Ca2+ to SR regulatory proteins. [source] Barotropic instability in the tropical cyclone outer regionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 641 2009Jiayi Peng Abstract The growth of asymmetric perturbations and their interactions with the symmetric flow are investigated for wind profiles in a tropical cyclone with instability in its outer region. Three tangential wind profiles are examined: TC1, a strong barotropic instability profile in the outer region; TC2, a stable wind profile; and TC3, a weaker instability profile comparing to TC1 with a larger distance between the inner negative and the outer positive vorticity gradient centres. An eigenvalue analysis indicates that azimuthal wave-number two is the most unstable mode in both TC1 and TC3, with an e-folding time-scale of about 1 and 9 days, respectively. Numerical simulations using a linear barotropic model, with an initial asymmetry specified in the outer region, confirm the eigenvalue analysis. A mechanism is provided to explain the difference between simulations in TC1 and TC2. In both the stable and unstable case, an inner asymmetry is induced by the initial outer asymmetry acting on the symmetric vorticity gradient. Subsequently, the newly generated inner asymmetry feeds back positively to the outer asymmetry with the unstable profile. Because of this positive feedback, the inner and the outer asymmetries maintain an up-shear phase tilting, leading to a continuous energy transfer from the symmetric flow to the asymmetric perturbation. In the stable TC2, the inner asymmetry could not amplify the outer initial asymmetry as there is no basic-state radial vorticity gradient there. Also due to this feedback process, disturbances grow faster where the (absolute) basic-state vorticity gradients are large. Therefore, the position of an initial disturbance plays a minor role in determining the outcome of the system. Simulations with a nonlinear barotropic model and a primitive equation model further confirm the significant weakening of the maximum tangential wind due to the positive feedback process in TC1. Simulations for TC3 show a smaller change of the symmetric tangential wind, as expected. Copyright © 2009 Royal Meteorological Society [source] Tropical-cyclone intensification and predictability in three dimensionsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 632 2008Nguyen Van Sang Abstract We present numerical-model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak-tropical-storm-strength vortex, the emergent flow becomes highly asymmetric and dominated by deep convective vortex structures, even though the problem as posed is essentially axisymmetric. The asymmetries that develop are highly sensitive to the boundary-layer moisture distribution. When a small random moisture perturbation is added in the boundary layer at the initial time, the pattern of evolution of the flow asymmetries is changed dramatically, and a non-negligible spread in the local and azimuthally-averaged intensity results. We conclude, first, that the flow on the convective scales exhibits a degree of randomness, and only those asymmetric features that survive in an ensemble average of many realizations can be regarded as robust; and secondly, that there is an intrinsic uncertainty in the prediction of maximum intensity using either maximum-wind or minimum-surface-pressure metrics. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of tropical cyclones, which may have a major impact on the intensity and on rapid intensity changes. Some other aspects of vortex structure are addressed also, including vortex-size parameters, and sensitivity to the inclusion of different physical processes or higher spatial resolution. We investigate also the analogous problem on a ,-plane, a prototype problem for tropical-cyclone motion. A new perspective on the putative role of the wind--evaporation feedback process for tropical-cyclone intensification is offered also. The results provide new insight into the fluid dynamics of the intensification process in three dimensions, and at the same time suggest limitations of deterministic prediction for the mesoscale structure. Larger-scale characteristics, such as the radius of gale-force winds and ,-gyres, are found to be less variable than their mesoscale counterparts. Copyright © 2008 Royal Meteorological Society [source] The dynamics of NAO teleconnection pattern growth and decayTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 589 2003Steven B. Feldstein Abstract This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ. Both high-frequency (period <10 days) and low-frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low-frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society [source] An Arctic and antarctic perspective on recent climate changeINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2007John Turner Abstract We contrast recent climatic and environmental changes and their causes in the Arctic and the Antarctic. There are continuing increases in surface temperatures, losses of sea ice and tundra, and warming of permafrost over broad areas of the Arctic, while most of the major increase in Antarctic temperatures is on the Antarctic Peninsula associated with sea ice loss in the Bellingshausen,Amundsen Seas sector. While both natural atmospheric and oceanic variability, and changes in external forcing including increased greenhouse gas concentrations, must be considered in the quest for understanding such changes, the interactions and feedbacks between system components are particularly strong at high latitudes. For the 1950s to date in the Arctic and for 1957 to date in the Antarctic, positive trends in large-scale atmospheric circulation represented by the Arctic oscillation (AO) and Antarctic oscillations (AAO) and the Pacific North American (PNA) pattern contribute to the long-term temperature trends. However, continuing Arctic trends during the last decade of near neutral AO will require alternate explanations. The trend in the AAO since 1950 is larger than expected from natural variability and may be associated with the decrease in stratospheric ozone over Antarctic. The persistence shown in many Arctic and Antarctic Peninsula components of climate and their influence through possible feedback supports continuation of current trends over the next decade. One can expect large spatial and temporal differences, however, from the relative contributions of intrinsic variability, external forcing, and internal feedback/amplifications. It is particularly important to resolve regional feedback processes in future projections based on modeling scenarios. Copyright © 2006 Royal Meteorological Society. [source] Radio constraints on the volume filling factors of AGN windsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009A. J. Blustin ABSTRACT The calculation of mass outflow rates of active galactic nuclei (AGN) winds is of great importance in understanding the role that such winds play in AGN-galaxy feedback processes. The mass outflow rates are, however, difficult to estimate since the volume filling factors of the winds are unknown. In this paper, we use constraints imposed by the observed radio emission to obtain upper limits to the volume filling factors of wind components in certain nearby AGN. We do this by predicting the 1.4 GHz radio flux densities emitted by those components, assuming a uniform wind, and then comparing these with the observed flux densities for each AGN at this frequency. We find that the upper limits to the volume filling factors are in the range of 10,4,0.5. [source] The nature of galaxy bias and clusteringMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2000A. J. Benson We have used a combination of high resolution cosmological N -body simulations and semi-analytic modelling of galaxy formation to investigate the processes that determine the spatial distribution of galaxies in cold dark matter (CDM) models and its relation to the spatial distribution of dark matter. The galaxy distribution depends sensitively on the efficiency with which galaxies form in haloes of different mass. In small mass haloes, galaxy formation is inhibited by the reheating of cooled gas by feedback processes, whereas in large mass haloes, it is inhibited by the long cooling time of the gas. As a result, the mass-to-light ratio of haloes has a deep minimum at the halo mass, ,1012 M,, associated with L* galaxies, where galaxy formation is most efficient. This dependence of galaxy formation efficiency on halo mass leads to a scale-dependent bias in the distribution of galaxies relative to the distribution of mass. On large scales, the bias in the galaxy distribution is related in a simple way to the bias in the distribution of massive haloes. On small scales, the correlation function is determined by the interplay between various effects including the spatial exclusion of dark matter haloes, the distribution function of the number of galaxies occupying a single dark matter halo and, to a lesser extent, dynamical friction. Remarkably, these processes conspire to produce a correlation function in a flat, ,0=0.3, CDM model that is close to a power law over nearly four orders of magnitude in amplitude. This model agrees well with the correlation function of galaxies measured in the automated-plate measurement survey. On small scales, the model galaxies are less strongly clustered than the dark matter, whereas on large scales they trace the occupied haloes. Our clustering predictions are robust to changes in the parameters of the galaxy formation model, provided only those models which match the bright end of the galaxy luminosity function are considered. [source] NGC 1275: Baryonic cycing in the Perseus cluster of galaxies,ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009J.S. Gallagher Abstract NGC 1275, the central galaxy of the Perseus cluster, contains a remarkable cool ISM. This gas is arrayed in a series of filaments extending over 30 kpc with an extension of 70 kpc to the north. These emit substantial cooling line radiation from HII, a hot H2 zone, and CO in a cool molecular region which contains most of the gas mass. This contribution briefly discusses some of the issues raised by the existence of this remarkable ISM system, including its connections to galactic feedback processes (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||||||||